fixed enter/leave execution order (it was run on the same workspace object)

d->current_workspace was updated before doing d->current_workspace->leave(), so it was leaving the newly created workspace before reentering it. This fixes issue on sdm-modeler where menu items were added to workspaces where they shouldn't be. Now, the previous workspace is correctly left by lauching its leave() method, removing its menu items, before the new workspace runs its enter() method.